DESCRIPTION

The functions select() and pselect() wait for a number of file
descriptors to change status.
Their function is identical, with three differences:
(i) The select() function uses a timeout that is a structtimeval
(with seconds and microseconds), while pselect() uses a structtimespec (with seconds and nanoseconds).
(ii) The select() function may update the timeout parameter to
indicate how much time was left. The pselect() function does not
change this parameter.
(iii) The select() function has no sigmask parameter, and behaves as
pselect() called with NULL sigmask.
Three independent sets of descriptors are watched. Those listed in
readfds will be watched to see if characters become available for
reading (more precisely, to see if a read will not block; in
particular, a file descriptor is also ready on end-of-file), those in
writefds will be watched to see if a write will not block, and those in
exceptfds will be watched for exceptions. On exit, the sets are
modified in place to indicate which descriptors actually changed
status.
Four macros are provided to manipulate the sets. FD_ZERO() will clear
a set. FD_SET() and FD_CLR() add or remove a given descriptor from a
set. FD_ISSET() tests to see if a descriptor is part of the set; this
is useful after select() returns.
n is the highest-numbered descriptor in any of the three sets, plus 1.
timeout is an upper bound on the amount of time elapsed before select()
returns. It may be zero, causing select() to return immediately. (This
is useful for polling.) If timeout is NULL (no timeout), select() can
block indefinitely.
sigmask is a pointer to a signal mask (see sigprocmask(2)); if it is
not NULL, then pselect() first replaces the current signal mask by the
one pointed to by sigmask, then does the ‘select’ function, and then
restores the original signal mask again.
The idea of pselect() is that if one wants to wait for an event, either
a signal or something on a file descriptor, an atomic test is needed to
prevent race conditions. (Suppose the signal handler sets a global flag
and returns. Then a test of this global flag followed by a call of
select() could hang indefinitely if the signal arrived just after the
test but just before the call. On the other hand, pselect() allows one
to first block signals, handle the signals that have come in, then call
pselect() with the desired sigmask, avoiding the race.) Since Linux
today does not have a pselect() system call, the current glibc2 routine
still contains this race.
Thetimeout
The time structures involved are defined in <sys/time.h> and look like
struct timeval {
long tv_sec; /* seconds */
long tv_usec; /* microseconds */
};
and
struct timespec {
long tv_sec; /* seconds */
long tv_nsec; /* nanoseconds */
};
(However, see below on the POSIX 1003.1-2001 versions.)
Some code calls select() with all three sets empty, n zero, and a non-
null timeout as a fairly portable way to sleep with subsecond
precision.
On Linux, the function select() modifies timeout to reflect the amount
of time not slept; most other implementations do not do this. This
causes problems both when Linux code which reads timeout is ported to
other operating systems, and when code is ported to Linux that reuses a
struct timeval for multiple select()s in a loop without reinitializing
it. Consider timeout to be undefined after select() returns.

RETURNVALUE

On success, select() and pselect() return the number of descriptors
contained in the three returned descriptor sets (that is, the total
number of one bits in readfds, writefds, exceptfds) which may be zero
if the timeout expires before anything interesting happens. On error,
-1 is returned, and errno is set appropriately; the sets and timeout
become undefined, so do not rely on their contents after an error.

ERRORS

EBADF An invalid file descriptor was given in one of the sets.
EINTR A non blocked signal was caught.
EINVALn is negative or the value contained within timeout is invalid.
ENOMEMselect() was unable to allocate memory for internal tables.

CONFORMINGTO

4.4BSD (the select() function first appeared in 4.2BSD). Generally
portable to/from non-BSD systems supporting clones of the BSD socket
layer (including System V variants). However, note that the System V
variant typically sets the timeout variable before exit, but the BSD
variant does not.
The pselect() function is defined in IEEE Std 1003.1g-2000 (POSIX.1g),
and part of POSIX 1003.1-2001. It is found in glibc2.1 and later.
Glibc2.0 has a function with this name, that however does not take a
sigmask parameter.

NOTES

An fd_set is a fixed size buffer. Executing FD_CLR or FD_SET with a
value of fd that is negative or is equal to or larger than FD_SETSIZE
will result in undefined behavior. Moreover, POSIX requires fd to be a
valid file descriptor.
Concerning the types involved, the classical situation is that the two
fields of a struct timeval are longs (as shown above), and the struct
is defined in <sys/time.h>. The POSIX 1003.1-2001 situation is
struct timeval {
time_t tv_sec; /* seconds */
suseconds_t tv_usec; /* microseconds */
};
where the struct is defined in <sys/select.h> and the data types time_t
and suseconds_t are defined in <sys/types.h>.
Concerning prototypes, the classical situation is that one should
include <time.h> for select(). The POSIX 1003.1-2001 situation is that
one should include <sys/select.h> for select() and pselect(). Libc4
and libc5 do not have a <sys/select.h> header; under glibc 2.0 and
later this header exists. Under glibc 2.0 it unconditionally gives the
wrong prototype for pselect(), under glibc 2.1-2.2.1 it gives pselect()
when _GNU_SOURCE is defined, under glibc 2.2.2-2.2.4 it gives it when
_XOPEN_SOURCE is defined and has a value of 600 or larger. No doubt,
since POSIX 1003.1-2001, it should give the prototype by default.

BUGS

pselect() is currently emulated with a user-space wrapper that has a
race condition. For reliable (and more portable) signal trapping, use
the self-pipe trick. (Where a signal handler writes to a pipe whose
other end is read by the main loop.)
Under Linux, select() may report a socket file descriptor as "ready for
reading", while nevertheless a subsequent read blocks. This could for
example happen when data has arrived but upon examination has wrong
checksum and is discarded. There may be other circumstances. Thus it
may be safer to use O_NONBLOCK on sockets that should not block.